Electric discharge lamps



July 18, 1967 J MOORE: ,ETAL

ELECTRIC DI SCHARGE LAMPS Filed Feb. 11, 1953 United States Patent 3,331,649 ELECTRIC DISCHARGE LAMPS James William Moore, Rugby, and Maurice George Clarke, Church Lawford, near Rugby, England, assignors to Associated Electrical Industries Limited, London, England, a company of Great Britain Filed Feb. 11, 1963, Ser. No. 257,403 Claims priority, application Great Britain, Feb. 13, 1962, 5,510/ 62 1 Claim. (Cl. 316-18) This invention relates to electric discharge lamps of the kind wherein the envelope comprises two members, at least one of which is wholly or mainly of vitreous material, one or both of the members being provided with an outwardly extending rim and at least one of the members being shaped to provide, when assembled with and sealed to the other member along a perimeter of the rim(s), a sinuous path for an electric discharge between electrodes located one at each end of the path. The discharge lamps With which the invention is mainly concerned are fluorescent lamps, wherein the light emission arises from the excitation of a layer of luminescent material provided on the inner wall of the envelope by radiation emitted from the discharge between the electrodes, although the invention is also applicable to discharge lamps in which visible radiation arises solely from the discharge between the electrodes.

Envelopes for electric discharge lamps of the kind with which the invention is concerned may be constructed from two glass pressings which are either the same as or mirror images of one another and which are so shaped as to provide a sinuous discharge path when they are assembled with their outwardly extending rims in contiguity and sealed along the perimeter thereof. Alternatively, one prmsing may be shaped to provide the sinuous discharge path, while the other member is in the form of a glass plate which is placed upon the pressing and sealed thereto along the perimeter of the outwardly extending rim of the pressing and the edge of the plate. The envelopes of such lamps as have hitherto been constructed are customarily of rectangular shape, so that the lamp, when energised, with a discharge taking place between electrodes located at the end of the sinuous path provides a substantially unifonnly illuminated panel.

In such lamps the question arises of the means to be adopted for evacuating and gas-filling of the envelope subsequent to the sealing together of the two members which constitute the envelope. If an exhaust tube is sealed into one of the vitreous members it is diflicult to seal off the exhaust tube without leaving a protuberance which may readily be damaged during use, thereby rendering the lamp inoperative. Furthermore, the necessity of sealing an exhaust tube to one of the members as a separate operation introduces an additional step during manufacture.

An object of the present invention is to provide a method of sealing an envelope of a lamp which avoids leaving a protuberance on the envelope after the sealing operation.

According to the present invention, an electric discharge lamp of the kind hereinbefore set forth has the envelope thereof sealed by a method which includes the steps of locating the members constituting the envelope together with a metal exhaust tube extending part way between the width of the rim of one of said members and the adjacent portion of the other member, heating the outer perimeter of the or each rim to a temperature suflicient to soften the vitreous material, allowing or urging the outer perimeters of the members to come into contiguity with one another and the exhaust tube to form a seal along the perimeter of the rim(s), evacuating the envelope, collapsing the vitreous material lying at the inner end of the exhaust tube and withdrawing the exhaust tube from between the members.

The metal or alloy of which the exhaust tube is composed should be one which is wet by and forms an hermetic seal with the vitreous material of the members so that the exhaust process can be carried out. If the envelope is allowed to cool after its periphery has been sealed and prior to it being evacuated, the coeflicient of thermal expansion of the metal or alloy should be matched with that of the vitreous material. The sealing of the envelope subsequent to evacuation is effected by collapsing the vitreous material lying at the inner end of the exhaust tube so as to complete the hermetic seal for the envelope and then withdrawing the exhaust tube from the sealed rim of the members.

The heating of the perimeter of the or each rim may be brought about by means of a gas flame, but with this arrangement it is necessary to move the flames and the members relatively to one another in order to heat the rims uniformly. This arrangement requires bulky supporting equipment and preferably the perimeter of the rim(s) is heated by a combination of an external heating source and the flow of electric current through the contiguous rim(s) of the members.

When the two members are in the form of pressings which are similar to mirror images of one another, the rim of each member may be provided with a raised portion which, when the members are assembled, forms an opening through which the metal exhaust tube may be inserted. In this way the rims of the two members may be contiguous over their entire length, except for the raised portion adjacent the exhaust tube. When the vitreous material is heated and softens it will sink into and form an hermetic seal with the metal exhaust tube, thus enabling the evacuation process to be carried out. If, however, the raised portion is not provided and the exhaust tube is inserted between adjacent flat rims of the members, a considerable portion of the perimeter of the rims will not be in contact with one another, and when the vitreous material has softened pressure must be applied to the rims to urge them into contiguity.

In order that the invention may be more readily understood, it will now be described, by way of example, with reference to the accompanying drawing which shows in perspective a discharge lamp with a portion of the envelope thereof broken away to illustrate a method of sealing the envelope in accordance with the present invention.

The envelope comprises two glass pressings 1 and 2 of generally rectangular form, and each pressing is formed with an outwardly extending rim 3. The pressings are also shaped to provide at least one internal barrier 4 which, when the two pressings are assembled with the rims 3 in contiguity, provide a sinuous path within the envelope. An electrode, one of which is shown at 5, is positioned at each end of the path, and when the electrodes are energised a discharge takes place along the sinuous path. The internal walls of the envelope including those of the barrier 4 may be coated with a luminescent material when the device is to operate as a fluorescent lamp.

To seal the two pressings together to form the envelope of the lamp, the pressings are assembled with their rims in contiguity and a metal exhaust tube 6 is located between the two rims, but extends only along part of the width of the rims 3 which lies between the edge of the pressing and the domed centre portion 7 thereof. The pressing 1 is provided with a raised portion 8 on the rim thereof to enable the exhaust tube to be inserted beneath the raised portion and between the two rims, thereby enabling the remaining portion of the rims to be in contact. If desired, both pressings may be provided with a raised portion which are so positioned that when the pressings are assembled together the raised portion forms an open- 3 ing into the envelope along part of which the exhaust tube extends.

A plurality of stationary gas jets 9 are located around the perimeter of the rims and an electrode 10 is positioned in contact with the outside edge of the rims at each corner of the pressings to enable the heating of the rims to be provided by a combination of gas heating and the flow of electric current through the rims of the pressing.

The method of heating the rims of the envelope involves providing an even number (at least four) of'electrodes to the contiguous rims of the pressings, and after preheating of the rims applying electrical power sequentially to pairs of sections of the current path which lie between the electrodes in such a manner that virtually no voltage exists across the remainder of the sections. The electrical power is supplied by way of a transformer 11 having a primary winding 12 and two secondary windings 13 and 14. One terminal of each secondary winding is interconnected with the movable member of a switching device 15, while the others, between which no voltage exists, are connected respectively to one of a pair of electrodes positionedat diagonally-opopsite corners of the envelope, and the other two electrodes, positioned on the rims of the pressings, are connected to fixed contacts of the switching device 15. When the movable contact of switching device 15 engages with one of the fixed contacts, electrical power is supplied along the rims on two sides of the pressings,

and when the movable contact engages with the other fixed contact power is supplied along the other two sides of the pressings. The frequency of change-over of the switching device may be once per second, thus alternating current flows from the two secondary windings of the transformer. 11, first through two sections of the current path around the pressings, and then through the other two sections.

- It is necessary to pro-heat the vitreous material to a temperature suflicient to render the material at the perimeter of the pressings electricaly conducting and to ensure that no cracking of the material takes place when the additional local heating is effected by the application of electrical power. When the vitreous material has been raised to sucha temperature that it softens, the two pressings will seal together along the continguous portions of the rims, but it may be necessary to apply pressure to one or both of the pressings to urge it or them into sealing engagement with the exhaust tube 6 in order that the envelope can be exhausted and filled with any inert gas that is required. Since the metal exhaust tube extends only partly across the width of the rims the effect of the exhaust tube is to provide a short-circuit for the electric power passed along the rims so that the vitreous material lying beyond the inner end of theexhaust tube is heated to a lesser extent than that in the vicinity of the exhaust tube itself, so that the vitreous material becomes less plastic than that at the edge of the rim and does not obstruct the inner end of the exhaust tube. On completion of the evacuation of the envelope the vitreous material lying at the inner end of the exhaust tube is caused to collapse, thereby completely sealing the envelope, and while the vitreous material at the edge of the rim is stillsoft the exhaust tube is withdrawn from the envelope. After the exhaust tube has been removed it may be desirable to shape the rims of the pressings between dies whilst the material is still soft in order to make the joint commercially presentable.

What we claim is:

A method of sealing the envelope of an electric discharge lamp wherein said envelope is of vitreous material comprising the steps of locating said members together 7 with a metal exhaust tube extending part way between the width of the rim of one of said members and'the adjacent portion of the other member, initially applying heat from an external source to the outer perimeter of each rim to soften the rims and to render the material electrically conductive, applying electric current to said rims in alternate paths to further heat saidn'ms, urging the outer perimeter of said members together, evacuating the envelope, collapsing the rim at the inner end of said exhaust tube and thereafter withdrawing said exhaust tube.

References Cited UNITED STATES PATENTS 3,117,248 1/1964 Lake 313187 3,240,861 3/1966 Christy 174 17.05

TRAVIS C. MCGEHEE, Primary Examiner.

FRANK E. BAILEY, GEORGE N. WESTBY, D. SRAGOW, Assistant Examiners. 

